This invention relates generally to a method for recovery of undersize crystalline particulates requiring hydrolysis of the particular crystalline material being employed and more particularly for the treatment of undersized fused magnesia particles in this manner in order to develop a larger particle size upon subsequent processing.
Fused magnesia particulates are conventionally produced in an electric arc furnace forming a solid ingot from the molten starting material which is thereafter mechanically crushed and ground by various means to a final desired particle size. The further conventional crushing and grinding processes being employed produces a particle size averaging around 30 mesh United States screen size accompanied by considerable magnesia dust having a particle size no greater than approximately 250 mesh United States screen size. The dust by-product cannot thereafter be reclaimed by refusion in the arc-type furnace being employed due to its light fluffy condition which effectively resists any further physical agglomeration as well as being too easily blown out of the fusion vessel by the hot gases accompanying furnace operation. The fine dust material also allows molten magnesia to migrate away from the hot zone in the fusion vessel which can result in damage of the vessel walls and with such damage to the fusion vessel further occasioning a violent expulsion of molten magnesia to the surroundings.
A wide assortment of fused magnesia products can be manufactured in the foregoing manner to include various types and grades for dissimilar end use applications. To further illustrate such diversity, this material is now commonly employed in electrical heating elements, refractory structures, and friction brake constructions. The electrical grade material for heating elements typically ranges in magnesia purity from at least 50 weight percent up to at least 98 weight percent, with a grain size ranging from approximately 20 to 40 weight percent in the size range -40 mesh United States screen size, while further exhibiting required electrical resistance and thermal conductivity characteristics. A still further requirement for said electrical heating element applications is mechanical flow of the fused magnesia particulates when filling the metal casings employed for said elements which can be enhanced by coating the magnesia particles with a solid or liquid lubricant, such as a silicone and the like. As distinct therefrom, a typical fused magnesia product for refractory structures such as bricks and blocks maintains a relatively high purity magnesia content in the 95-98 weight percent range together with a particle size range from 80-2,000 microns. The physical properties commonly specified for fused magnesia products to replace asbestos in friction brake constructions includes high temperature stability and Mohs hardness of at least 5. To satisfy these requirements, a typical product ranges in magnesia content from approximately 90-99 weight percent with a particle size in the -40+60 mesh United States screen size from approximately 25-35 weight percent.
A fused magnesia product suitable for electrical heating elements can be produced in the above manner utilizing a conventional submerged arc electric furnace of the general type long employed for steel-melting. Electric heating is supplied with single or multiple electrodes to establish a reaction zone within the fusion vessel where the mineral charge becomes melted. In doing so, fusion can commence upon a batch charge already present in the furnace vessel with additional mineral batches being charged during the fusion process. When the charged material has been converted to a molten state, the applied electrical power is terminated allowing the furnace contents to cool providing a solid ingot with varying degrees of crystalline formation. The furnace ingot is subsequently crushed by conventional mechanical means such as with a jaw crusher followed by a hammer mill which can still further include magnetic separation means to remove any entrained metal contaminants. A conventional vibrating screen apparatus is next employed to provide the desired particle size from the crushed material while further yielding at least 5 weight percent undersize particles passing through 325 mesh United States screen size. The desired particle size material is then heated in an oxidizing atmosphere to approximately 1000.degree. C. for additional impurity removal and optionally blended with a suitable lubricant to provide a free-flowing powder having the above defined electrical and physical characteristics suitable for heating element utilization. Accordingly, it remains desirable to recover the undersized fused magnesia particles resulting from the aforementioned crushing and grinding processes so as to reduce environmental problems caused by dust escape in the manufacturing plant as well as significantly increase the yield of usable product being manufactured.
It is one object of the present invention, therefore, to provide effective means whereby undersize fused magnesia particulates can be converted to larger size particles.
It another object of the present invention to provide novel treatment means enabling fused magnesia particles no greater than approximately 250 mesh United States screen size to be effectively reprocessed into a larger particle size polycrystalline product.
Still another object of the present invention is to provide a novel method for recovery of undersized fused magnesia particulates so as to increase product yield from the starting material.
These and still further objects of the present invention will become apparent upon considering the following detailed description of the present invention.